Conventionally, in optical communication, wavelength division multiplex (WDM) networks that transmit WDM optical signals are used (see, for example, Japanese Laid-Open Patent Publication No. 2010-056676). In WDM networks, optical add-drop devices such as optical add-drop multiplexers (OADM) and reconfigurable-OADMs (R-OADM) that add and/or drop optical signals according to wavelength are used.
Functions for efficiently and flexibly building, modifying, and managing WDM networks is demanded of optical add-drop devices. For example, functions enabling wavelength dependency (colored), direction dependency (directional), and collision of identical wavelengths (contention) among nodes to be avoided is demanded of optical add-drop devices. Such functions are called colorless, directionless, contentionless (CDC) functions.
Colorless describes a function of being able to input an arbitrary wavelength into an arbitrary port of an optical add-drop device and to output an arbitrary wavelength from an arbitrary output. Directionless describes a function of being able to guide optical signals from terminals to an arbitrary degree and to guide optical signals from the degrees to an arbitrary terminal, in a configuration in which the optical add-drop device has multiple degrees. Contentionless describes a function of being able to avoid collisions of optical signals of the same wavelength in the optical add-drop device.
However, the conventional technology above has a problem in that optical signal amplification cannot be efficiently performed. In particular, in a configuration realizing CDC function, since optical elements are numerous, an amplifier is used along the path to compensate optical signal loss. Further, in the configuration realizing CDC function, since optical elements of multiple ports are connected, the number of times an optical signal is dropped and the number of input paths, as well as the number of amplifiers for compensating optical element loss, become numerous.
In particular, among paths that drop channels, optical signals may concentrate at a particular path. Moreover, since the path where the optical signals concentrate is flexibly changed according to application, the path where the optical signals concentrate cannot be preliminarily designated. Consequently, if each of the amplifiers is given the functional capacity to sufficiently perform amplification when optical signals concentrate, amplifier cost becomes high. Further, in an amplifier of a path where optical signals do not concentrate, the capability of the amplifier is not fully utilized, making the utilization rate of the amplifier low.